1. Field of the Invention
The present invention relates to a key sheet for push-button switches to be used in an operation section of various kinds of devices such as a mobile phone, a PDA, a car navigation system, and a car audio system. In particular, the present invention relates to a key sheet suitable for the use of a plurality of key tops from an operation opening formed in the case of the device with no partition bridge.
2. Description of the Related Art
Like a mobile phone 1 shown in FIG. 32, due to a desire for miniaturization of the whole device or an operation section thereof and a desire for design performance, and so on, it has been desired to provide push-button switches such that plural key tops 3 of a key sheet 2 are narrowly arranged and exposed from an operation opening 1b formed in a case 1 a with no partition bridge. As shown in FIG. 33, the key sheet 2 of the related art is constructed of a plurality of key tops 3 (i.e., 17 key tops 3 in total) firmly fixed on a base sheet 4 made of silicone rubber. That is, the key sheet 2 includes a large-sized key top 3a in the middle upper side for input in upward, downward, right, and left directions in FIG. 33, four small-sized key tops 3b located on the left and right sides of the key top 3a, and twelve middle-sized key tops 3c located below the key tops 3b. The distance between the adjacent key tops 3a, 3b, and 3c is extremely narrow. For example, they are narrowly arranged with a distance of about 0.15 mm to 0.2 mm. They are also extremely narrowly spaced from the operation opening 1b and the distance is almost the same as the above.
The structure for mounting such a key sheet 2 is constructed such that structural elements inside the case 1a (in this prior art, the opening-edge portion of the operation opening 1b in the back side 1c of the case 1a and a circuit board 1d built in the case 1a) compress and retain completely around the outer edge portions of a base sheet 4. The inside of a pressure-contact portion is provided with an installed structure that does not restrain the case 1a and the circuit board 1d. Thus, when the key sheet 2 is turned upright as shown in FIG. 35 or is turned upside down as shown in FIG. 36 at the time of using the mobile phone 1, the base sheet 4 made of a rubber-like elastic body such as flexible silicone rubber may be extended and distorted as a whole by the weight loading of the key top 3. When the key sheet 2 is distorted on the whole in this manner, a pusher 4a of the base sheet 4 and a contact switch 1e constructed of a metal disk spring and a contact circuit in a circuit board 1d are displaced from each other. Thus, poor operation such as the impossibility or difficulty in input even by depressing the key top 3 may occur. In addition, depending on the appearance of the distorted base sheet 4, the amount of an input stroke differs for every key top 3, so that it may affect the operational performance. Furthermore, there is a problem in that the mobile phone 1 looks unattractive. Besides, one of the adjacent key tops 3 may slip into the space under the other.
The problems caused by distortion of the base sheet 4 made of the flexible rubber-like elastic body as described above should be solved particularly with respect to the key sheet 2 having all of the narrowly-arranged key tops 3 exposed from the operating opening 1b as shown in the figure. However, these problems may occur when one operation opening is formed for the key tops 3b located at the upper and lower positions (i.e., when two or more key tops are arranged in a single operation opening). In addition, even when a key sheet with a plurality of narrowly spaced key tops is mounted to a device when in use that is not designed to be turned upright or turned upside down as the mobile phone 1, problems such as slippage of one key top under the adjacent key top may occur as a result of distortion of the base sheet made of a flexible rubber-like elastic body. Therefore, the countermeasure is also demanded about the key sheet.